Many electrical devices today, which may be ICs or embedded cores within ICs, include a JTAG (IEEE 1149.1) Test Access Port to provide access to test, debug, emulation, and/or programming circuitry within the device. One thing that makes the JTAG Test Access Port attractive for use in a device is that its interface signals, consisting of a Test Data Input (TDI), Test Mode Select (TMS), Test Clock (TCK) and Test Data Output (TDO), are dedicated and therefore available for use at any stage of the device's lifetime, i.e. manufacturing through end use in a system. Since the JTAG Test Access Port became an IEEE standard in 1990, other IEEE standards (IEEE 1149.4, IEEE 1149.6 and IEEE 1532) have been developed based on the JTAG Test Access Port and signal interface. These other IEEE standards are compliant to the rules in the JTAG Test Access Port standard to insure interoperability between a device incorporating the JTAG Test Access Port standard and a device incorporating the other standards.
During development of the JTAG standard it was anticipated that the dedicated Test Access Port interface signals mentioned above may need to be used for testing purposes that are not compliant to the JTAG standard. To prepare for this possibility, the JTAG standard set forth rules and permissions in the standard to allow a compliance enable pattern to be input to a device, via additional signal inputs, to enable the JTAG interface signals to be used for other testing purposes.
The present disclosure provides a method and apparatus for allowing a device's JTAG interface signals to be selectively used for; (1) accessing the device's JTAG Test Access Port, (2) accessing JTAG compliant Access Ports, (3) accessing JTAG compatible Access Ports, and (4) accessing non-JTAG Access Ports. Advantageously, the access port selection method and apparatus of the disclosure is achieved using only the JTAG standard interface signals TDI, TMS, TCK and TDO.
FIG. 1 illustrates a first example of a standard JTAG Test Access Port (TAP) 102 in a device 104. The JTAG TAP includes a TDI input, a TMS input, a TCK input, an optional TRST input, a TDO output, and an output enable (OE) output. TDI inputs data to the TAP, TMS inputs control to the TAP, TCK inputs clocks to the TAP, and TDO outputs data from the TAP. The OE output is used to enable a device output buffer to output the TDO output from the TAP whenever the TAP is in the Shift-DR or Shift-DR states of the TAP state diagram of FIG. 4.
FIG. 2 illustrates a second example of a standard JTAG Test Access Port (TAP) 102 in a device 202. The JTAG TAP of FIG. 2 is exactly the same as the JTAG TAP of FIG. 1. The only difference is that the TRST input to the JTAG TAP is provided by a power on reset (POR) circuit within the device 202 instead of by the optional TRST input.
FIG. 3 illustrates the architecture of the standard JTAG TAP 102. The architecture includes a JTAG TAP controller 302, an instruction register 304, a bypass register 306, one or more data registers 308, multiplexers 310 and 312, and a TDO output registration flip flop (FF) 314. The TAP controller 302 controls the capturing, shifting and updating of data to the instruction register, bypass register, and data registers from TDI to TDO. The instruction register 304 stores an instruction that selects data to be shifted through the single bit bypass register or through a selected data register. The data registers 308 provide data input and data output to test and/or other circuits in the device. The multiplexers 310 and 312 pass the output of the selected register (instruction, bypass, or data) to the TDO output via FF 314. The architecture and operation of the standard JTAG TAP 102 is well known in the industry.
FIG. 4 illustrates the state diagram that defines the operation of the JTAG TAP controller 302 of FIG. 3. State diagram transitions occur on the rising edge of TCK in response the logic level on TMS, as shown in FIG. 5. Also as seen in FIG. 5 and during the Shift-DR and Shift-IR states, data is made available on TDI for input to the JTAG TAP from an external source and data is made available on TDO for output from the JTAG TAP to an external source on the rising edge of TCK. The timing and operation of the TAP state diagram is well known in the industry.